Radioactive .sup.14 C is constantly being generated through the interaction of cosmic rays with the upper atmosphere. This radioactive carbon, in the form of CO.sub.2, enters the "carbon cycle" as plants utilize CO.sub.2 in the production of food. .sup.14 C is constantly decaying with a half-life of approximately 5730 years. In the atmosphere, where .sup.14 C is constantly replenished, there is a constant ratio between the amount of .sup.14 C and the other isotopes of carbon. When a living organism dies, the organic matter which forms the organism no longer interacts with the atmosphere. Thus, over time, as the radioactive C.sup.14 decays, the ratio between .sup.14 C and the non-radioactive isotopes of carbon changes and this change in the ratio can be used to determine how long an assemblage of organic material has been dead.
Monitoring the radioactive decay of .sup.14 C in a sample of dead organic matter allows the calculation of the amount of time which has passed since the plant or animal from which the organic matter is derived died.
Although monitoring the decay of .sup.14 C is an effective method of dating organic matter, the process has several limitations which can render the process inaccurate or impractical. Because of the relatively low radioactivity--less than fourteen disintegrations per minute per gram of carbon--of organic derived carbon, sample sizes on the order of a gram are required for the conventional process. Another limitation is that the samples are destroyed during the dating process. The smaller the sample the greater the amount of time required to determine the amount of .sup.14 C present in the test sample by monitoring radioactive disintegrations. The older the sample the more sensitive the age-determination is to a precise measurement of the .sup.14 C present. Many ancient artifacts are of such value that only extremely small samples can be submitted for destruction in the .sup.14 C dating process.
The foregoing physical and practical limitations mean that .sup.14 C dating by conventional methods is of limited practical value.
To overcome these problems a mass spectrometer can be used to determine the amount of .sup.14 C present in a sample, directly. Because all the .sup.14 C atoms in a sample are available for direct detection, the ratio between .sup.14 C and the other isotopes in a carbon sample can be rapidly determined for extremely small samples with a high degree of precision.
There are a number of practical problems associated with the use of a mass spectrometer to determine the isotope ratios between .sup.14 C and the other isotopes of carbon. The first step of the process whereby a mass spectrometer may be used to determine the relative abundance of .sup.14 C in a sample is the creation of a negative ion beam containing carbon ions from the sample. Once a negative ion beam of carbon atoms has been produced a tandem accelerator can be used to separate carbon ions from all other molecular isomers. The isolated carbon ions are then analyzed by use of a mass-spectrometer which measures the amount of .sup.14 C present in the sample relative to the other isotopes of carbon.
The tandem accelerator, the ion beam processing equipment and the ion detectors make up an apparatus of such cost and size that there exist relatively few facilities housing them.
On the other hand, the number of scientists, particularly archaeologists, for whom .sup.14 C is a critical part of the work they do, is relatively large. Each article for which it is desired to determine a .sup.14 C date must be separately sampled and tested. The sample sizes available are typically on the order of thousandths of a gram. The small sample sizes are in part due to the historical value of the artifact, for example the Dead Sea Scrolls. Small sample size can also be the result of the attempt to date a non-organic artifact from a small amount of organic material found in association with the artifact. For example, the dating of successive layers of soil in an archaeological dig by the dating of charcoal residue from campfires to thereby establish a date for all the artifacts buried in each layer of soil at the dig.
Thus, there is an ever increasing demand for a more accurate processing of more samples. In order for a sample's age to be determined by mass spectrometry .sup.14 C dating, it must be converted to a beam of negative ions. This is typically done by converting the sample to carbon dioxide gas. The gas is absorbed onto a titanium cathode where a cesium ion beam interacting with the carbon dioxide absorbed on the surface of the titanium generates negative carbon ions. An alternative approach is to form graphite by depositing carbon from the carbon dioxide gas onto a substrate and bombarding the graphite with cesium ions. As the process is currently practiced, the scientist or the laboratory takes a sample of carbon-containing material and converts it through combustion to carbon dioxide gas, which is sealed in a glass bottle. The samples, in the form of carbon dioxide gas, are then sent to a facility having a properly equipped tandem accelerator where the .sup.14 C analysis is performed.
.sup.14 C dating, like most data gathering procedures, is subject to technique induced errors. In the past, the samples of carbon dioxide have been withdrawn from the ampule into a transfer chamber or syringe and then transferred by way of a tube to the cathode of the ionization chamber. Contamination from the air or with organic matter will result in an understatement of the age of the sample being tested.
The processes whereby .sup.14 C samples are dated with a mass spectrometer are relatively new. The normal progress in improving the reliability of a new scientific procedure is for the individual researchers to become intimately involved in performing and developing standard techniques by which a given analysis is performed. This approach is not practical in mass spectrometry .sup.14 C dating because of the relatively small number of facilities capable of performing the test. Current practice has evolved to the point where multiple samples are sent to different .sup.14 C mass spectrometry analysis facilities to improve the perceived reliability of the numbers.
While .sup.14 C dating may seem of mere academic or historical interest, its use, in combination with modem genetic testing, can play a key role in setting environmental policy. Questions of whether an animal population is losing genetic diversity can be solved with reference to museum collections of specimens whose accurate dating by .sup.14 C, together with genetic material recovered from preserved pelts and animal specimens, can provide data on historic change or lack thereof regarding genetic diversity in a studied population.
Recently, for example, a controversy has arisen as to whether the red wolf is a distinct species of animal or whether it is merely a cross between coyotes and grey wolves. Light has been shed on this question through research and age dating of furs contained in the Smithsonian Institution's fur collection.
What is needed is a way of reducing or eliminating technique-induced errors between the generation of a carbon dioxide gas sample and its introduction into the ion source utilized in the mass spectrometric determination of the .sup.14 C dating process.